Use recommended defaults for sbt to avoid memory problems.

Default brew install of `sbt` easily runs out of memory.

The addition of these settings `-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M` can prevent this from happening.

Settings were taken from http://www.scala-sbt.org/release/docs/Getting-Started/Setup.html.

Based from answers in http://stackoverflow.com/questions/15957183/outofmemoryerror-from-sbt.